Overview of the Underpinnings and Building Blocks of Soil Taxonomy Soil Forming Processes Overview of the Underpinnings and Building Blocks of Soil Taxonomy

Why is this topic important? Associating soil morphology with pedogenic process will make you a better pedologist An appreciation of these scientific underpinnings is required in order to understand rationale for the concepts in Soil Taxonomy

Objectives List Simonson’s 4 general processes of soil formation Understand purpose of the diagnostic horizons in Soil Taxonomy Know the difference between “diagnostic horizons” and “horizon designations (e.g., A-Bt-Bk-C)”

Generalized Theory of Soil Genesis (Simonson, 1959) Processes There are many possible physical, chemical, and biological reactions and interactions that occur in soils. These are categorized as soil-forming processes. The possible number of process events, their combinations, and the interactions among them is impressive. Processes of soil formation occur in unique proportions, intensities, and sequences. The horizons of a soil profile reflect the relative strength of these processes and the degree to which they offset each other. Parent Material (Phys, Chem., Biological) “biogeochemical” Today Time ‘0’

Additions Removals Transfers Transformations Physical, chemical and biological reactions and interactions are categorized as soil-forming processes Additions Removals Transfers Transformations ASK THE CLASS FOR EXAMPLES OF EACH OF THE 4 PROCESSES Possible answers: Additions - OM accumulation, CaCO3 and sodium salt dust, groundwater recharge of bases, etc. Removals - Soil erosion, Leaching of bases and soluble salts, OM oxidation, etc. Transfers - Eluviation/illuviation (e.g. clay, AL, Fe, humified OM, silica, CaCO3), base cycling, etc. Transformations - Mineral weathering (e.g. muscovite > vermiculite, vermiculite > illite, feldspar > montmorillonite, vitric volcanic glass > cristobalite), Fe+3 > Fe+2, raw OM to humified OM, etc.

(Bockheim and Gennadiyev, 2000) Bockheim’s processes Simonson’s general processes Defined Base cation leaching removals eluviation of Ca, Mg, K, and Na from the solum under extreme leaching conditions Argilluviation transfers the movement of clay in the solum Paludization additions, transformations the deep ( >40 cm ) accumulation of organic soil materials Gleization transformations presence of reduced matrix or redoximorphic features such as concentrations, depletions, strong gleying, etc. Bockheim’s processes Taxa in Soil Taxonomy displaying process Base cation leaching Spodosols; Dystric great groups of Inceptisols and Vertisols, Dystric subgroups of Entisols; Ultisols; Ultic subgroups of Alfisols, Andisols, Entisols, Inceptisols, Mollisols, and Spodosols Argilluviation Alfisols; Ultisols; Argids suborder and Argic great groups of Aridisols; Albolls suborder and Argic great groups of Mollisols; Kandic great groups of Oxisols; Alfic subgroups of Spodosols; Argiorthels great group of Gelisols Paludization Histisols; Histels suborder and Histic great groups of Gelisols Gleization Aquic suborders of most orders (except Aridisols, Gelisols, and Histosols); Aquic great groups of Aridisols and Gelisols (Bockheim and Gennadiyev, 2000)

Objectives List Simonson’s 4 general processes of soil formation Understand purpose of the diagnostic horizons in Soil Taxonomy Know the difference between “diagnostic horizons” and “horizon designations”

Where Jenny’s soil forming factors (clorpt) are the same: (Guy Smith, 1963) Where Jenny’s soil forming factors (clorpt) are the same: genetic expression is similar [result of soil forming processes] the soils are the same due to pedogenesis Similar soils respond equally to the same management. Soil genesis is that part of soil science that deals with factors and processes of soil development; the science of evolution of soils. Soil genesis is a discipline that has survived the fragmentation of soil science into soil chemistry, physics, mineralogy, and fertility. It can be thought of as a system of bridges that connect eight islands called chemistry, physics, geology, biology, climatology, geography, anthropology, and agriculture. We conduct soil surveys to help us understand how best to manage soils and predict behavior from their properties.

Measure the morphological expression of soil genesis (Guy Smith, 1963) If soils w/ similar genetic expression respond the same, our goal is to: Have a taxonomy that groups soils with similar genesis But you can’t observe soil genesis, so what’s the next best thing? Measure the morphological expression of soil genesis Soil genesis is like an experiment that has already been conducted. The soil scientist is left to observe the result and interpret the pedogenic processes that formed the soil. Guy Smith - Genesis does not appear in the definitions of the taxa but lies behind them.

Spodosol Diagnostic Horizons Processes Morphology Ochric epipedon - Addition of organic matter Ochric epipedon - Removal of base cations by leaching Albic horizon - Transformations to form amorphous alumino-organic complexes Spodic horizon - Transfer of alumino-organic constituents and iron from surface to the subsurface

Diagnostic Horizons Taxonomic standards that reflect our understanding of soil genesis Definitions are quantitative; objective. Focus is on properties, not theories of genesis Any competent soil scientist can classify Based on measurable & tactile soil properties that are present today. Not presumed for native state Identified partly by their own morphology and how their properties compare to those in the overlying and underlying horizons Not all mutually exclusive. 2. Diagnostic horizons were developed by the authors of Soil Taxonomy to solve the problem of the subjective nature of horizon designations. There was then and still is today, lack of agreement on what type or degree of genesis a particular horizon designation implies. The strict definitions of the diagnostic horizons allow soils to be classified based on their properties and not on the assumed genetic processes which formed them.

Not all mutually exclusive Mollic epipedon Argillic horizon Not all mutually exclusive Mollic epipedons and argillic horizons are not mutually exclusive and their properties can occur overlap in the same subhorizon. Ochric epipedons and cambic horizons are also not mutually exclusive. However, mollic epipedons and cambic horizons are mutually exclusive and cannot overlap in the same subhorizon. Ochric epipedons and argillic horizons are also mutually exclusive.

Objectives List Simonson’s 4 general processes of soil formation Understand purpose of the diagnostic horizons in Soil Taxonomy Know the difference between “diagnostic horizons” and “horizon designations”

Terminology Horizon designations Opportunity for the describer to express their opinion of soil genesis (A, Ap, Bt, Btk, Bk, Bkqm, C, etc.) based on what they see or infer Horizon designations are subjective, based on the describer’s experience/understanding of soil-forming factors and processes

Spodosol Diagnostic Horizons Processes Morphology - Addition of organic matter A Ochric epipedon - Removal of base cations by leaching E Albic horizon Same soil profile shown previously with symbol designations for the genetic horizons added. The A and E horizons constitute the ochric epipedon. The E horizon has albic materials and constitutes the albic horizon. The Bhs and Bs horizons constitute the spodic horizon. Bhs Spodic horizon - Transformations to form amorphous Alumino-organic complexes Bs C - Transfer of alumino-organic constituents and iron from albic to spodic

Terminology Horizon designations Therefore, DO NOT ASSUME horizon designation implies a diagnostic horizon Some examples: Oa ≠ histic epipedon Bk ≠ calcic horizon Bt ≠ argillic horizon Bw ≠ cambic horizon Bx ≠ fragipan

Diagnostic Horizons and Horizon Designations Definitions are measurable/observable Only 1 correct answer Horizon Designations Definitions less objective Reflects describers theory of genesis

Summary Horizons are morphological expressions of soil-forming processes Diagnostic horizons are objectively defined taxonomic entities with strict criteria Horizon designations are subjectively assigned and reflects the describer’s theory of soil genesis